Sunday, March 30, 2014

A paper published today in Nature finds "clear" evidence of a "hydrologic seesaw in which latitudinal migrations of the Intertropical Convergence Zone (ITCZ) produce simultaneous wetting (increased precipitation) in one hemisphere and drying in the other." According to the authors, these shifts can have "serious effects on temperate climate systems. Furthermore, our result implies that insolation-driven ITCZ dynamics may provoke water vapour and vegetation feedbacks in northern mid-latitude regions and could have regulated global climate conditions throughout the late Quaternary ice age cycles."

Glacials and interglacials on the northern and southern hemisphere somehow do not seem to correspond. This has led to a ‘thermal bipolar seesaw theory,’ whereby an off-mode in the thermohaline circulation leads to an ice age in Europe, but excess heat storage down south.

Given that ice cores show a lag between Arctic and Antarctic climate changes of 300-1500 years, it begs the question given that Antarctic sea ice is at record highs and Arctic sea ice has declined during the satellite era, whether this may represent part of a natural seesaw pattern, shifting heat via the thermohaline circulation from the Southern Hemisphere to the Northern Hemisphere and explaining why less warming has been observed in the Southern Hemisphere since 1850.

Mid-latitude interhemispheric hydrologic seesaw over the past 550,000 years

An interhemispheric hydrologic seesaw—in which latitudinal migrations of the Intertropical Convergence Zone (ITCZ) produce simultaneous wetting (increased precipitation) in one hemisphere and drying in the other—has been discovered in some tropical and subtropical regions. For instance, Chinese and Brazilian subtropical speleothem (cave formations such as stalactites and stalagmites) records show opposite trends in time series of oxygen isotopes (a proxy for precipitation variability) at millennial to orbital timescales, suggesting that hydrologic cycles were antiphased in the northerly versus southerly subtropics. This tropical to subtropical hydrologic phenomenon is likely to be an initial and important climatic response to orbital forcing. The impacts of such an interhemispheric hydrologic seesaw on higher-latitude regions and the global climate system, however, are unknown. Here we show that the antiphasing seen in the tropical records is also present in both hemispheres of the mid-latitude western Pacific Ocean. Our results are based on a new 550,000-year record of the growth frequency of speleothems from the Korean peninsula, which we compare to Southern Hemisphere equivalents. The Korean data are discontinuous and derived from 24 separate speleothems, but still allow the identification of periods of peak speleothem growth and, thus, precipitation. The clear hemispheric antiphasing indicates that the sphere of influence of the interhemispheric hydrologic seesaw over the past 550,000 years extended at least to the mid-latitudes, such as northeast Asia, and that orbital-timescale ITCZ shifts can haveserious effects on temperate climate systems. Furthermore, our result implies that insolation-driven ITCZ dynamics may provoke water vapour and vegetation feedbacks in northern mid-latitude regions and could have regulated global climate conditions throughout the late Quaternary ice age cycles.

Theoretical models and observational data have long suggested that the Northern and Southern Hemisphere climates behave in a seesaw-like fashion: when the northern ocean warms, the southern ocean cools and vice versa. So far, however, the data have indicated a much muted response in Antarctic climate compared to the Arctic. An analysis of new records from an ocean core from the South Atlantic — including planktonic foraminifera assemblages, Mg/Ca ratios, temperature and ocean productivity data — shows that the South Atlantic cooled essentially instantaneously with the warming in the North Atlantic during the last deglaciation. This first concrete evidence of an immediate seesaw connection also provides a link between the rapid warming in the North Atlantic and the more gradual Antarctic response, and suggests a mechanism potentially driving rapid Northern Hemisphere deglaciation.

NEWS AND VIEWS:Climate change: Southern see-saw seen

The bipolar see-saw hypothesis provides an explanation for why temperature shifts in the two hemispheres were out of phase at certain times. The hypothesis has now passed a test of one of its predictions.